The present invention relates to a system for measuring the quantity of an intake air in an engine for a motor vehicle, in order to determine the fuel injection time and the ignition timing.
In a known fuel injection system, an airflow meter is provided in an intake passage at a position upstream of a throttle valve to detect the quantity of intake air (Q). The fuel injection time Tp is determined by a calculation of Tp=Q/N (N is engine speed).
Further, the ignition timing is also determined by using the quantity Q of intake air. Accordingly, a high accuracy is required in the measurement of quantity of intake air. However, in the prior art, it is difficult to accurately measure the quantity of intake air as described hereinafter.
Since the airflow meter is positioned upstream of the throttle valve, the intake air induced into a cylinder of the engine is not directly measured. Namely, there is a space between the throttle valve and the cylinder, such as a chamber formed downstream of the throttle valve and the intake manifold. Accordingly, when the throttle valve is rapidly opened, the induced air enters into the space to increase the pressure in the space. In other words, an amount of air sufficient to increase the pressure in the space is included in the air induced through the throttle valve. The airflow meter measures the quantity of the intake air including the air which is not induced into the cylinder. Accordingly, the measured quantity increases temporarily. If the injection time is calculated based on the increased quantity of intake air, the fuel injector supplies a larger quantity of fuel than a necessary quantity, thereby enriching the air-fuel mixture. The enrichment of the mixture causes a reduction of the output of the engine and increases of noxious emissions such as CO and HC. Further, if the ignition timing is calculated based on the increased quantity of the intake air, the timing deviates from a desired timing.
Similarly, when the throttle valve is closed, the air-fuel ratio and ignition timing deviate from desired values.